Castle sidekicks Seamus Dever and Jon Huertas are no strangers to the stage. Not only does Jon have his own music career on the side ("Sex is the Word," anyone?), but the pair also enjoy putting their voices together for a good cause.Last March, Seamus and Jon busted out Warren G's "Regulate" at UNICEF's Playlist for the A List event, and this past weekend, they slowed things down at The Anti Cabaret Cabaret with Stevie Wonder's "Lately." With Seamus on the piano and Jon on lead vocals, the dynamic duo got plenty of chuckles from the audience at M Bar in Los Angeles. Even Seamus's wife Juliana joined in on the fun. "I'm not sure, but I think Jon and Seamus just sang each other a break up song," she tweeted during the show.Too funny!Watch Seamus and Jon's hilarious performance below: